1. Field of the Invention
The invention is in the fields of combustion and gas turbines, and especially gas turbine designs for dry low emissions, and more particularly concerns apparatus and methods for premixing fuel and air to achieve ultra low combustion emissions.
2. Description of the Related Art
A premixer can be useful in enhancing flame stability in an energy release/conversion system, such as a combustor for powering a gas turbine engine or generator. For purposes of brevity, this disclosure uses the term “combustor” throughout, but it should be understood that the disclosure relates more generally to energy release/conversion systems that can be operated as either a combustor or a fuel reformer, depending on operating conditions, as well as dedicated combustor and reformer apparatus. Unless the context clearly requires otherwise, the terms “combustor”, “reformer” and “energy release/conversion system” should be regarded as completely interchangeable.
Premixers currently employed in the art include “hub and spoke” configured units, such as those employed by General Electric, Pratt & Whitney, Siemens, etc., placed at the inlet of a combustor, with fuel supplied through the hub and injected out of radial spokes and/or integrated into swirler vanes. The spokes of these premixers have a plurality of uniform sized axial holes transverse to the combustor inlet. The design of these premixers tends to optimize them for a particular fuel-air momentum flux ratio. Accordingly, these premixers work best in a narrow power band, and do not provide the most uniform fuel-air mixture over the entire engine operating envelope. The system would generate either too lean or too rich fuel-air zones, adversely impacting emissions. It appears, therefore, that further improvement in the operation of energy release/conversion systems might be possible by redesigning the premixing apparatus used in conjunction with these systems to perform better over a broader range of operating conditions.
In one class of combustion apparatus, known as “trapped vortex” combustors (TVCs, as addressed at further length later in this disclosure), a cavity is provided in the combustion area, for example, between bluff bodies or in the wall of the combustor, in which vortices and/or other turbulence will form, in order to stabilize combustion with lean mixtures. See for example U.S. Pat. No. 5,857,339 to Roquemore et al. Fuel and/or air may be injected into a trapped vortex cavity through discrete injectors, to induce greater mixing in this area and further promote flame stability. The discrete fuel and air injectors may be situated, for example, on the forward and aft walls of a trapped vortex area defined by the walls of the combustion cavity. See, e.g., Burrus, U.S. Pat. No. 5,791,148.
Haynes, et al., GE Global Research, “Advanced Combustion Systems for Next Generation Gas Turbines, Final Report”, January 2006 (DE-FC26-01NT41020), describes a combustor similar in layout to the combustor described by Burrus. In certain embodiments disclosed by Haynes et al., as an alternative to discrete fuel and air inlets, as previously practiced, fuel and air may be premixed and introduced through the inlet cone and/or through the forward or aft walls of the combustion cavity. Embodiments in which premixture was introduced both into the inlet cone and the combustion cavity resulted in the creation of a stacked double vortex, with highly turbulent mixing.
Steele, et al., in U.S. Pat. No. 7,603,841, describing another TVC embodiment, discloses a combustor having inlet premixing as well as aft injectors into a combustion cavity defined in part by a bluff body. In this embodiment, the aft injectors are pointed in a direction opposite to incoming premixture flow to induce turbulent vortex mixing.
Heretofore, all TVC designs that have injected fuel, air and/or premixed fuel and air into the combustion cavity have been designed to induce turbulence, to cause the formation of additional vortices, or otherwise to increase turbulent mixing in the vortex cavity. For example, FIG. 3-7 of Haynes et al. shows a dual trapped vortex in each TVC cavity. The “natural” flow that would occur in these cavities, meaning the fluid flow that would naturally occur in the cavities in the absence of the premixture injection, given the flow otherwise taking place through the main flow path of the combustor, would be a single vortex. In the case shown in FIG. 3-7 of Haynes et al., the second vortex in the “dual vortex” show is created by the injection of premixture into the TVC cavity, and would not otherwise exist. In other cases, for example, where only a single vortex is provided, the main vortex shown might otherwise exist, but is substantially modified by the impact of the added premixture, for example, by being moved translationally from its natural position in the cavity, made much more turbulent, or otherwise substantially distorted.
Commonly assigned U.S. Pat. Pub. 2008/0092544 A1, by Rakhmailov (Rakhmailov '544 publication), discloses a premixer deployed in combination with a combustor designed in accordance with the disclosure of commonly assigned U.S. Pat. No. 7,086,854 to Rakhmailov et al. (Rakhmailov '854). The premixer in the Rakhmailov '544 publication is deployed only at the inlet of the combustor. The inlet of this combustor has a high velocity of fluid flow, and thus the inlet premixing is done in a high-velocity environment.
While the design described in the Rakhmailov '544 publication adds an inlet premixer to the recirculating vortex combustor described in Rakhmailov '854, neither disclosure contains any provision for injection of fuel, air and/or premixed fuel and air directly into the vortex cavity. Indeed, Rakhmailov '854 expressly teaches away from admitting fuel into the hot recirculating gas in a recirculation vortex cavity, stating that turbulent mechanical mixing can reduce overall recirculation velocity, result in nonuniform fuel distribution, and reduce temperatures where the recirculating flow joins the inlet flow, contrary to the design goals of Rakhmailov '854.
The entire respective disclosures of Roquemore et al., Burris, Haynes et al., Steele et al., Rakhmailov '854, and the Rakhmailov '544 publication are each incorporated by reference herein in their entirety, for all purposes.
It would be desirable to improve upon the prior art in a number of respects. First, it would be desirable to improve inlet premixers for any type of combustor by making the premixer more adaptable to a wider range of operating conditions. Second, it would be desirable to provide for premixing in the vortex area of a combustor to enhance rather than disrupt normal vortex flow. Third, it would be desirable to provide methods for using inlet and vortex premixers advantageously in combination with each other.